Fragile X Syndrome (FraX) is a developmental brain disorder with abnormal neuron architecture development and functional plasticity of the developing brain, causing mental retardation, learning disabilities and autism. My lab established the Drosophila FraX model and proved it provides direct insights into molecular and cellular bases of the human disease state. In this revised proposal, I ask your support to take advantage of this model, and the relative simplicity of the Drosophila brain, to test core hypotheses of FraX and interventions to correct brain defects. First, I will test the temporal requirements for the Fragile X Mental Retardation Protein (FMRP) in defined brain neural circuit formation and the maintained manifestation of normal behaviors from these circuits. I target two extremely well-defined brain circuits;1) the circadian clock circuit that regulates regular motor activity cycles, and 2) the mushroom body (MB) circuit that mediates olfactory learning/memory consolidation. This aim will characterize the development of these circuits and their synaptic connectivity in the presence vs. absence of FMRP. A transgenic conditional FMRP expression system will be used to test the hypothesis that FMRP is required specifically during a transient period of neural circuit development to establish maintenance of normal behaviors. Transgenic animals expressing FMRP during discrete developmental windows will be tested for anatomical neural circuit formation, synaptic structure and function, and the behavior outputs of circadian activity and olfactory learning and memory consolidation. Second, I will test the hypothesized role of FMRP in sensory modality, electrical and synaptic neurotransmission activity-dependent changes in brain neural circuit development. The interaction between FMRP function and brain circuit activity will be assayed in double mutant combinations and with an array of proven transgenic tools that either increase or decrease electrical activity in targeted brain regions. The specific role of metabotropic neurotransmitter receptor signaling will be assayed in both cholinergic and glutamatergic brain regions using a combination of genetic mutants and pharmacological studies. The temporal roles of these pathways will be assessed with timed application of drugs during defined stages of brain development to define temporal windows for therapeutic intervention. The role of FMRP in local translation control downstream of a neurotransmission activity-induced phosphorylation will be tested by making transgenic constitutively phosphorylated or desphosphorylated mimic proteins. It is imperative to appreciate that these hypothesized functions have never been tested in vivo, in an animal model. Third, I will use new proteomic technologies to screen for brain protein changes occurring during specific periods of brain development, in the presence and absence of FMRP. In parallel, systematic forward genetic screens will be pursued to directly identify dfmr1 genetic interactors. Together, these aims are designed to make maximal use of the proven Drosophila FraX genetic model. I am the only one poised to pursue this work and truly believe that I can aid enormously in the understanding and treatment of Fragile X Syndrome. PUBLIC HEALTH RELEVANCE: Fragile X Syndrome is a developmental brain disorder that is the most commonly inherited form of both mental retardation and autism spectrum disorders. This proposal studies the molecular and cellular basis of this disease in the developing brain and directly tests therapeutic interventions aimed at correcting the brain developmental abnormalities.